Reproductive Behaviour of Japanese Tubesnout, Aulichthys japonicus (Gasterosteiformes), in the Natural Habitat Compared with Relatives

We observed significant sexual dimorphism in size (females > males) and in colour in Japanese tubesnout, Aulichthys japonicus, in Aburatsubo Bay, central Japan, and Otsuchi Bay, northern Japan. This species is known to spawn its eggs into ascidians. We observed reproductive behaviour of tubesnout in the natural environment in both bays. All the males established territories at harbours or rocky shores and showed conspicuous nuptial colouration with metallic green or blue on their snout and bright yellow on the body. Females had a dark brown body with remarkable white spots. The male territories varied somewhat in location and extent on a daily basis. The individual male size and territory size were not correlated. The females usually stayed in the seaweed beds where no males were present. Males and females were distributed separately, and the gravid females visited the male territories in groups to spawn. The territorial males courted the gravid females by rushing and returning, and tried to lead the females to the particular place of their territories by tail beating. We discuss nuptial colouration, courtship behaviour and territoriality in comparison to the related species; Aulorhynchus flavidus, Hypoptychus dybowskii, and gasterosteid fishes.